[BUG] [DefaultCodeGen] Map<String, X> is generated as API requestBody if Model class has additionalProperties=true

Description

In case a model class has additionalProperties=true, in the API a Map is inserted as requestBody parameter instead of the model class.

openapi-generator version

4.0.3

OpenAPI declaration file content or url

See steps to reproduce.

Command line used for generation

See steps to reproduce.

Steps to reproduce

I used the following pom.xml to generate the sources: https://gist.github.com/balazs-zsoldos/7c707b098d97bac59a3b411d5d6b0664

For an example, see GroupResourceAPI.addUserToGroup(...). The requestBody parameter here is a Map, but it should be UpdateUserToGroupBean.

Related issues/PRs

Suggest a fix

I analyzed the problem and I think the behavior could be changed in DefaultCodeGen.fromRequestBody(...) function, more specifically changing this and the next line to the following code solves the issue:

if (name != null) {
    CodegenModel codegenModel = fromModel(name, schema);
    codegenParameter.baseType = codegenModel.classname;
    codegenParameter.dataType = getTypeDeclaration(codegenModel.classname);
    imports.add(codegenParameter.baseType);
} else {
    codegenParameter.dataType = getTypeDeclaration(schema);
    codegenParameter.baseType = getSchemaType(inner);
}

However, I am not sure how this would affect all generators that already exist. If there is a specific reason why Map is inserted in this case instead of the model type. I would like to send a patch that does not cause any trouble.

Do you think this is a good fix? Or shall I create a new configuration property for the generator like useModelTypeForRequestBodyParamWhereAdditionalPropertiesIsTrue and make it default false? In that case, the current behavior is not changed only if explicitly sets this configuration to true.

Start with DefaultCodegen.java and inspect DefaultCodegen.fromRequestBody(...) around the linked lines, then reproduce the generated GroupResourceAPI.addUserToGroup(...) case using the pom.xml gist. Check how the change affects existing generators; done means the request body uses UpdateUserToGroupBean rather than Map without regressions across relevant generator tests.